1. Field of the Invention
This invention relates to a catalyst carrier with excellent refractory properties and befitting a high-temperature reaction such as catalytic combustion.
2. Description of the Prior Art
The method of catalytic combustion which effects fuel combustion through the assistance of a catalyst allows the combustion to proceed at a lower temperature and depresses the occurrence of themal NO.sub.x as compared with the conventional method of combustion without the use of a catalyst and, as such, has been attracting keen attention recently from the standpoint of prevention of air pollution.
Incidentally, since the temperature of the reaction caused by the method of catalytic combustion exceeds 1,000.degree. C., the catalyst to be used for the reaction must possess refractory properties, and particularly, the catalyst carrier serving to retain a reactive component in a highly dispersed state is required to possess these high refractory properties. The catalyst carrier has specific surface area as one of the important qualities. For the catalyst carrier to retain the reactive component in a highly dispersed state, it is necessary that the specific surface area of the catalyst carrier should be sufficiently large. The catalyst carrier with high refractory properties, therefore, must be a carrier capable of retaining a large specific surface area even at high temperatures. Specifically, it is required to retain a specific surface area exceeding 10 m.sup.2 /g.
At present, a carrier formed mainly of .gamma.-alumina is used most widely as a refractory carrier possessing a large specific surface area. The .gamma.-alumina, however, undergoes thermal transformation into .alpha.-alumina accompanied by abrupt sintering and loss of specific surface area i.e. to the extent of ceasing to function as part of a carrier, when it is heated to 1,200.degree. C.
As a solution to this problem, the present inventors have already proposed a catalyst carrier for high-temperature combustion which comprises alumina and boron oxide.
At temperatures in the neighborhood of 1,200.degree. to 1,300.degree. C., this catalyst carrier can be effectively used because the specific surface area of this carrier is slightly under 30 m.sup.2 /g at 1,200.degree. C. and somewhere between 10 to --m.sup.2 /g at 1,300.degree. C. An attempt to use this catalyst carrier in the neighborhood of 1,400.degree. C., however, fails because the carrier undergoes sintering with a serious loss of its specific surface area to 4 to 5 m.sup.2 /g.
The objective of this invention is to provide a catalyst carrier which is used optimally for catalytic combustion even in the neighborhood of 1,400.degree. C.